Scientists have outlined how alkali treatment can make delignified wood highly transparent, as well as why transparency differs significantly between tangential and radial directions.
Study: Anisotropic Transparency of Alkali-Treated Wood. Image Credit: Shazam Shabeer/Shutterstock.com
A recent paper published in Macromolecular Materials and Engineering takes a closer look at the transparency enhancement mechanism in alkali-treated wood, addressing a long-standing gap in understanding how structural anisotropy influences optical performance.
Why Transparency Mechanisms Matter
Wood is naturally opaque and brown due to lignin and the light-scattering lumens within its cellular structure. In recent years, however, transparent wood has emerged as an advanced biomaterial with promising optical and mechanical properties.
It is typically produced by impregnating delignified white wood with transparent polymers or by thermally deforming and pressing wet brown wood at elevated temperatures. In some cases, transparency can also be achieved through pulp fiber nanofibrillation without polymer impregnation.
Additional chemical strategies, such as ionic liquid treatment, dialdehyde oxidation, and lytic polysaccharide monooxygenase (LPMO) oxidation, have further expanded the processing toolkit.
These approaches have enabled applications ranging from chemiluminescent systems to electromagnetic shielding and device circuit boards. Yet as processing methods multiply, a more fundamental question remains: what exactly governs transparency in wood, and why do certain treatments outperform others?
Many post-treatment techniques, including ionic liquid treatment, LPMO oxidation, and TEMPO oxidation, enhance transparency and density through capillary-driven self-densification. Alkali treatment has also been shown to improve transparency, particularly after drying, but its underlying mechanism has not been clearly defined.
Importantly, wood is not structurally uniform. As an orthotropic material, its properties vary along three mutually perpendicular directions. Boards cut from the same log - cross-sectional, radial, and tangential - exhibit distinct mechanical behaviors because of differences in cellular architecture. This directional dependence suggests that transparency is not controlled by chemistry alone. Instead, any complete explanation must account for how chemical modification interacts with wood’s intrinsic anisotropic structure.
It is at this intersection (chemical treatment and structural directionality) that the present study is focused.
Investigating the Mechanism: Linking Chemistry and Directionality
Building on the idea that transparency depends on both chemical modification and structural orientation, the researchers focused on the cellulose microfibril skeleton, the load-bearing framework that remains after lignin removal. Their goal was to understand how alkali treatment improves transparency and how this interacts with the anisotropic framework.
For the study, the team tested two closely related hypotheses.
First, that alkali treatment softens the cellulose microfibril skeleton by altering its chemical environment. Second, that the inherent anisotropy of this skeleton governs how the material swells, densifies, and ultimately transmits light in different cutting directions.
To evaluate these ideas, native balsa wood was immersed in distilled water overnight and cut into 30 × 30 × 2 mm samples. Tangential and radial sections were prepared, each with densities between 0.08 and 0.12 g/cm3, allowing the team to directly compare direction-dependent behavior under identical treatment conditions.
Five samples were delignified in a sealed Petri dish using acetic acid, distilled water, and sodium chlorite at 80 °C for 24 hours. After thorough washing, the delignified wood was immersed in 5 % potassium hydroxide (KOH) for 24 hours at room temperature. This alkali step was central to the study, as it was expected to remove additional hemicellulose and modify ionic interactions within the cell wall.
Drying was carefully controlled to capture structural changes during densification. Each sample was placed on a silane-treated glass plate, covered with absorbent paper and metal mesh, and dried at 30 °C under 5 kPa pressure for two to three days before storage at 13 % relative humidity.
To further clarify the role of refractive index matching and isolate structural effects, additional samples underwent stepwise ethanol immersion before being infiltrated with acrylic resin containing a photoinitiator. Because the resin’s refractive index closely matches that of cellulose, polymer impregnation reduces interfacial light scattering. This allowed the researchers to distinguish between transparency improvements caused by structural densification and those caused by optical matching.
Chemical composition analysis quantified hemicellulose, cellulose, and lignin content before and after treatment. Structural and optical changes were characterized using Fourier transform infrared spectroscopy (FTIR), optical microscopy, and ultraviolet–visible spectroscopy.
By designing the experiments this way and comparing radial and tangential sections under identical chemical conditions, the researchers positioned themselves to directly observe how alkali-induced softening and anisotropic structure work together to influence transparency.
Findings: How Softening and Anisotropy Converge
With radial and tangential sections subjected to identical chemical treatments, the differences that emerged could be traced directly to structural directionality rather than processing variation. The results showed that transparency is not governed solely by chemistry - it is the interaction between alkali-induced softening and anisotropic architecture that determines the final optical outcome.
After delignification, both sections remained translucent. Although lignin had been removed, preserved lumens continued to scatter light, limiting transmittance. At this stage, the cellulose microfibril skeleton remained relatively rigid, supported by residual hemicellulose and H+ counterions within the cell wall.
The shift occurred during alkali treatment.
Immersion in KOH removed additional hemicellulose and replaced carboxyl-group counterions with K+. This ion exchange altered the internal bonding environment of the cellulose framework, increasing hydrophilicity and mechanical compliance. In practical terms, the cell walls became softer and more deformable.
That softening proved decisive during drying. As water was removed under controlled pressure, the now-compliant cellulose microfibril skeleton densified more effectively. Lumens collapsed, void spaces decreased, and light-scattering interfaces were reduced. The result was highly transparent wood, even without polymer impregnation.
To isolate the role of ion exchange, the researchers compared cellulose nanofiber films containing H+ and K+ counterions.
Films with K+ exhibited significantly greater swelling, confirming that counterion substitution governs dimensional response and mechanical softening. This provided direct evidence that transparency enhancement stems from chemical modification of the cellulose framework rather than from capillary-driven effects alone.
Structural anisotropy then determined how that softening translated into optical performance.
Although radial and tangential sections initially exhibited similar transmittance, clear differences appeared after alkali treatment. The tangential section showed a lower thickness swelling ratio (4.1 %) compared with the radial section (13.3 %). As a result, vessel elements in the tangential section deformed into flattened cavities, while those in the radial section formed more elliptical cavities.
These distinct deformation modes influenced how effectively light-scattering voids were minimized. Tangential sections achieved greater densification and, consequently, higher total light transmission than radial sections.
Even when polymer impregnation was introduced to suppress interfacial scattering through refractive index matching, optical anisotropy persisted. Section thickness and deformation geometry continued to affect overall transmittance, reinforcing the conclusion that direction-dependent structure remains a governing factor, even after optical interfaces are optimized.
Taken together, the findings demonstrate that alkali treatment enhances transparency through a self-densification process driven by cell wall softening and counterion exchange.
Crucially, the extent and optical impact of this densification depend on the wood’s intrinsic anisotropic architecture.
Conclusion
This study clarifies how alkali treatment enhances the transparency of delignified wood. Rather than relying on capillary-driven effects, the improvement stems from hemicellulose removal and counterion exchange, which soften the cellulose microfibril skeleton and enable greater densification during drying.
At the same time, the results show that transparency is inherently direction-dependent. Tangential sections deform in a way that reduces light-scattering voids more effectively than radial sections, leading to higher overall transmittance.
By linking chemical modification with structural anisotropy, the work provides a clearer framework for designing transparent wood materials with predictable, direction-specific optical performance.
Journal Reference
Yagyu, H. et al. (2026). Anisotropic Transparency of Alkali-Treated Wood. Macromolecular Materials and Engineering, 311(2), e00389. DOI: 10.1002/mame.202500389, https://onlinelibrary.wiley.com/doi/10.1002/mame.202500389
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